Semicolons are used to separate related independent clauses that could stand on their own as sentences. For example, instead of each clause standing alone as a separate sentence, a semicolon can connect the following two independent clauses: "I can't go on a date with him; I have a boyfriend." Writers use semicolons to add variety and avoid having a number of short sentences.
Semicolons are often used to emphasize that two statements or clauses are related, so putting a semicolon between two unrelated clauses can lead to confusion. For example, "The dog was huge; I went for a walk" doesn't make sense, since the semicolon suggests that these two clauses should relate. If the second clause becomes "we went for a walk" instead, then the semicolon would let the reader assume that the word "we" refers to the writer and the dog.
One important consideration when using semicolons is the use of coordinating conjunctions. When joining two clauses, there is no need to use both a semicolon and a conjunction. In the following sentence, for instance, either the semicolon or the word "because" should be eliminated: "I can't go on a date with him; because I have a boyfriend."
Finally, semicolons can be used to separate items in a complex list. For example, "Tom, who is tall; Ted, who is short; Bill, who is skinny" would be confusing if the items in the list were separated by commas, so the semicolon helps keep the information clear and organized.